The present invention relates generally to the field of alarm systems for detecting intrusion into one or more locations and more specifically to the field of intrusion detection systems including mechanisms for displaying the particular location where an intrusion has occurred.
Many intrusion alarm systems employ a string of series-connected normally closed, single-pole-single-throw switches as intrusion sensors. A switch is placed at each protected port of entry such as a door, window or gate. A voltage is applied across this network so that a small current flows through the loop. If any switch is opened, either momentarily or continuously, such as by opening a door or window, the interrupted current is instantly detected by the system and an audible alarm is initiated. In a typical application, the leads from each switch or group of switches are brought to a common junction box or terminal strip and connected in series so that each switch is electrically accessible from a common point. Systems have been proposed, such as that described in U.S. Pat. No. 4,118,700, for indicating which of the sensor switches of the alarm system have operated, thus indicating an intrusion in the area protected by that particular sensor switch. A basic problem with such prior art alarm display systems is that they require the use of a resistor placed in parallel with each of the sensor switches in the series chain of sensor switches, the values of each of the resistors being a predetermined multiple of the previous resistor in the chain. Due to this requirement, it can easily be envisioned that in an embodiment requiring a large number of sensor switches, the parallel resistors at the lower end of the series chain may have such a large value as to render the concept impractical. Further, such systems require the use of analog-to-digital converters which are relatively expensive and add greatly to the cost of the overall system.